The present invention relates to a low-viscosity, hard polypropylene wax, to a process for its preparation and to its use. The thermal degradation of isotactic polypropylene gives hard PP waxes, but is energy-intensive and leads to sometimes discolored products which contain double bonds.
The polymerization of propene using Ziegler-Nafta catalysts (EP-A-584 586, DE-A-23 29 641) requires large amounts of hydrogen and is therefore likewise restricted to relatively high-viscosity waxes.
The preparation of 1-olefin polymer waxes which have a narrow molecular weight distribution and a high isotacticity by means of metallocenes is known. However, the resulting products continue to have a melt viscosity of 100 mPas even when regulation is carried out by means of a high hydrogen level (EP-A-321 852).
EP-A-321 853 describes the use of metallocene catalysts for preparing low molecular weight PP waxes. However, the resulting products have a low isotacticity of less than 70%.
Furthermore, there are metallocenes (EP-A-416 566) which have been used for preparing polypropylene of different molecular weight depending on the polymerization temperature. The mean molecular weight decreases as the polymerization temperature increases. The products prepared by this method have unsaturated chain ends and therefore have little thermal stability in use.